1. Field of the Invention
This invention relates to a method for preparing magnetic recording media.
2. Background Art
If magnetic paint used in the manufacture of magnetic recording media contains non-dispersed matter or reagglomerated matter of magnetic particles, etc. or resin insolubles, they become defects in the resulting magnetic coatings, incurring increased dropouts and increased error rates. Then a step of filtering magnetic paint through a filter is generally involved prior to coating. With respect to filtering in the magnetic paint preparation process, a variety of proposals have been made as mentioned below.
Japanese Patent Application Kokai (JP-A) No. 240830/1990 discloses in Examples that since magnetic paint as kneaded generally contains non-dispersed matters of ferromagnetic powder, binder and other ingredients and foreign matters such as abraded powder from the balls and milling container, the paint is passed through a filter immediately after kneading to effect filtration for removing such contaminants, and that the paint having subjected to dispersion treatment in an AC dispersion means or sand mill is passed through a filter for removing abraded powder resulting from the sand mill and non-dispersed matter. However, no reference is made to the pore size and other details of the filter
JP-A 133405/1978 discloses passage of magnetic paint through filter paper (pore size 1 .mu.m and 4 .mu.m) upon its coating. This patent publication describes merely a pore size and no specific definition of the pore size is described therein.
JP-A 143118/1979 discloses a method comprising the steps of filtering a binder, solvent, solution and the like except for a pigment consisting of magnetic powder through a filter having a mesh size not greater than the particle size of the pigment (of the order of 0.05 to 5 .mu.m), dispersing the pigment in the solution, then filtering the dispersion through a filter having a mesh size equal to or greater than the particle size of the pigment, and coating the magnetic paint onto an aluminum substrate.
JP-A 147134/1982 discloses a method comprising the step of removing agglomerates of magnetic powder and fractures of milling balls from magnetic paint by centrifugal separation, followed by filtration through a membrane filter having an average pore size of at least 5 .mu.m. It is noted that the definition of an average pore size is described nowhere in this patent publication.
JP-A 102332/1983 discloses a method of reducing bit errors by passing magnetic paint through a metallic filter (pore size 2 .mu.m or less) and then coating it to a substrate.
JP-A 58631/1984 discloses an apparatus for circulating magnetic paint in a paint tank by passing it through a filter disposed outside the tank. The pore size of this filter is described nowhere.
JP-A 117737/1984 discloses a method of reducing bit errors by kneading and dispersing magnetic paint, then passing it through a filter having a pore size of 3 .mu.m while agitating it for stabilization, and thereafter filtering it through a membrane filter having an average pore size of 20 .mu.m.
JP-A 82331/1986 discloses the manufacture of audio tape having a magnetic layer of 4.5 .mu.m thick by passing magnetic paint through a filter having pores of 3 to 5 .mu.m for filtration, followed by coating and calendering.
JP-A 248230/1986 discloses a method comprising filtering magnetic paint with concurrent ultrasonic treatment, followed by coating. Although the use of a filter of 10 .mu.m is described, the definition of a pore size is unclear.
JP-A 122927/1986 discloses a method comprising cooling magnetic paint to below about 20.degree. C., passing it through a filter, restoring to room temperature and then coating. An apparatus allowing plural passage of magnetic paint through a filter is disclosed in FIG. 2 of this patent publication although the pore size of this filter is described nowhere.
JP-A 6440/1987 discloses a method comprising the steps of passing a magnetic liquid through a first filter built-in ultrasonic tank to effect filtration concurrently with ultrasonic dispersion, and passing a portion of the magnetic liquid through a second filter built-in ultrasonic tank to effect filtration concurrently with ultrasonic dispersion again, or passing a portion of the magnetic liquid through a ultrasonic tank to effect dispersion again and then passing the some through a filter to effect filtration again. The patent publication discloses an apparatus for circulating the magnetic liquid through a filter although the pore size of this filter is described nowhere.
JP-A 134827/1987 discloses a method comprising the steps of previously preparing an abrasive dispersion containing an abrasive and a binder, and thereafter mixing the dispersion with ferromagnetic powder. The patent publication discloses that the magnetic paint is filtered through a filter having an average pore size of 0.5 to 1.5 .mu.m before it is coated. In Examples, using a filter having an average pore size of 1 .mu.m, 8-mm video tape having a thickness of 3 .mu.m after drying was manufactured. Using a 8-mm video recorder, the number of dropouts was counted to find 5 dropouts/minute. It is noted that the definition of an average pore size is described nowhere.
JP-A 258716/1987 discloses a filter apparatus comprising a housing containing a filter for filtering magnetic paint and an agitating blade disposed therein and adapted to rotate in proximity to a filter medium. The medium of the filter used in Examples disclosed therein is constructed of cellulose plus epoxy resin with a nominal removal rate of 2 to 5 .mu.m. It is not clearly understood what is meant by the nominal removal rate described therein.
JP-A 217528/1988 discloses a method comprising passing magnetic paint through an internal entrapment type filter having a mesh size of 40 to 100 .mu.m under gravity for removing gel-like non-dispersed matter and coarse foreign matter and filtering the paint through a precision filter, followed by coating, the method ensuring removal of the gel-like non-dispersed matter. The pore size of the precision filter is described nowhere in the patent publication.
JP-A 106331/1989 discloses a magnetic recording medium comprising first and second magnetic layers formed on a surface of a non-magnetic substrate in this order wherein the second magnetic layer has a thickness of less than 0.5 .mu.m and an abrasive contained in the second magnetic layer and having a Mohs hardness of at least 6 has a mean particle size of up to 0.5 .mu.m, whereby electromagnetic properties and running durability (dynamic durability) are improved. In Examples described therein, magnetic tapes were prepared by filtering a coating liquid for the first magnetic layer and a coating liquid for the second magnetic layer each through a filter having an average pore size of 1 .mu.m and then coating the second magnetic layer to a dry thickness of 0.25 .mu.m, 0.45 .mu.m or 1 .mu.m (Comparative Example). Electromagnetic properties (Y-S/N at 4 MHz) and running durability (still life) were measured. It is noted that the definition of an average pore size is described nowhere.
JP-A 87326/1990 discloses the manufacture of VHS cassettes or 8-mm video cassettes by passing magnetic paint through a filter of 1 .mu.m mesh and coating the thus prepared paint to a dry layer thickness of 3 .mu.m.
JP-A 270125/1990 discloses a magnetic recording medium comprising two magnetic layers, the upper layer having a thickness of up to 0.5 .mu.m and containing perpendicularly oriented ferromagnetic metal powder particles having a particle length of up to 0.1 .mu.m and an aspect ratio major axis/minor axis) of more than 1 and up to 3. In Examples described therein, magnetic recording media for 8-mm video application were prepared by filtering a paint for the upper layer and a paint for the lower layer each through a filter having an average pore size of 1 .mu.m and then forming the upper layer to a thickness of 0.29 .mu.m or 0.5 .mu.m. It is noted that the definition of an average pore size is described nowhere.
JP-A 3116/1991 discloses the filtration of magnetic paint through a filter wherein by adjusting the temperature of the magnetic paint to the range of 10.degree. to 20.degree. C., the flow and flexibility of gel-like non-decomposed matter and undissolved matter are reduced so that they may be fully captured by the filter. This patent publication describes a prior art technique of filtering magnetic paint through a filter having an absolute filtration precision of about 1 to 30 .mu.m. The patent publication discloses an apparatus of circulating a portion of the thus filtered magnetic paint to a pool tank for effecting filtration again although the pore size of a filter for this filtration is described nowhere. Also the definition of an absolute filtration precision is described nowhere.
Meanwhile, magnetic recording media require the magnetic layer at its surface to be smooth and highly stable during operation to comply with wavelength shortening and density increase of record signals. Employed to this end is a method of coating a backcoat layer to a back surface remote from the magnetic layer-bearing surface. If magnetic paint for the backcoat layer contains resin insolubles or non-dispersed matter and re-agglomerated matter of commonly added conductive pigment and various abrasives and solid impurities entrained during the manufacturing process, they, after formation of a backcoat layer, become defects in the backcoat layer surface to detract from the dynamic travel of magnetic recording media and the smoothness of the magnetic layer surface, thus incurring an increase of dropouts and a rise of error rate. Then with respect to filtering of paint for the backcoat layer, a variety of proposals have been made as mentioned below.
Japanese Patent Publication (JP-B) No. 79052/1992 contains an Example wherein VHS format video tapes were prepared by filtering paint for a backcoat layer through a filter having an average particle size of 0.5 .mu.m before and after viscosity adjustment, and forming a backcoat layer of 1 .mu.m or 0.6 .mu.m thick using this paint. These video tapes were measured for dropouts. However, the definition of an average particle size of the filter is described nowhere and the definition of the criterion for rating dropouts is described nowhere. In this patent publication, no filtration is made on paint for a magnetic layer.
JP-A 69833/1985 describes that a paint composition for a backcoat layer having a dispersion liquid and a crosslinking agent added and agitated for viscosity adjustment is coated onto a sheet of polyethylene terephthalate (PET) by means of an applicator to thereby form a backcoat layer of 0.2 to 0.5 .mu.m thick. The paint is filtered through filter paper prior to coating. Data regarding filtration precision such as the pore size of filter paper are described nowhere in this patent publication. In this patent publication, VHS tape having a backcoat layer formed thereon is measured for defect factor. The defect factor used herein is the number per minute of defects with which a level drop of at least 20 dB continues for 15 .mu.m or more and is generally known as a dropout. In Examples described in this patent publication, the defect factor is 5 to 10. No filtration is made on paint for a magnetic layer in this patent publication.
As mentioned above, no precise investigations have been made on the relationship between the properties of a magnetic recording medium and the properties of a filter used in preparing a paint used for the manufacture of the magnetic recording medium. For both the magnetic paint and backcoat layer-forming paint, most prior art techniques carry out filtration on those paint compositions containing a pigment as typified by magnetic powder while the one and only technique of carrying out filtration on a binder solution excluding a pigment is the one disclosed in JP-A 143118/1979.